In the field of motor control, a conventional motor starter typically consists of a contactor and a motor overload relay. The contactor is typically a three-pole switch, which is usually operated by a continuously energized solenoid coil. Since the contactor controls the operation of the motor, i.e., the starting and stopping, it must be rated for many thousands of operations.
The overload relay, on the other hand, provides overload protection to the motor from overload conditions. Overload conditions occur when equipment is operated in an electrically undamaged circuit in excess of normal full-load rating, or when conductors carry current in excess of rated ampacity. Overload conditions persisting for a sufficient length of time will damage or overheat the equipment. Overload conditions do not include faults, which require instantaneous protection such as a short circuit or ground fault or a loss of a phase. The terms xe2x80x9coverload,xe2x80x9d xe2x80x9coverload protectionxe2x80x9d and xe2x80x9coverload relayxe2x80x9d are defined in the National Electrical Manufacturers Association (NEMA) standard ICS2, which is herein incorporated by reference.
To protect a motor from faults, which require instantaneous protection, circuit breakers, e.g. instantaneous trip circuit breakers, are typically used. Additionally these circuit breakers function as a manual disconnect switch (disconnect) which serve to isolate the motor during a maintenance operation.
Devices which combine the instantaneous protection of a circuit breaker as well as the motor starter functions in a single enclosure are known as combination starters. However, the current-carrying components of instantaneous trip circuit breakers are constructed of heavy copper bars and large-sized tungsten contacts to protect against thermal damage upon short circuit interruption. This is especially so when the circuit breakers are used in circuits containing electric induction motors. The large size of the components increases the size of the circuit breaker to the extent that such circuit breakers do not fit within certain standard Asian and European circuit breaker enclosures. Therefore prior art combination starters utilizing instantaneous trip circuit breakers are bulky and expensive.
U.S. Pat. No. 4,754,247 entitled xe2x80x9cMolded Case Electric Switch Accessory Enclosurexe2x80x9d describes an arrangement whereby electric switch accessories can be inserted within the electric switch enclosure for electrical connection with the electric switch control circuit. U.S. Pat. No. 4,884,164 entitled xe2x80x9cMolded Case Electronic Circuit Interrupterxe2x80x9d describes an industrial-rated circuit breaker having an electronic trip unit that is adjustable over a wide range of long and short time over-current conditions. U.S. Pat. No. 4,149,216 entitled xe2x80x9cFused Unitized Combination Starterxe2x80x9d describes the early use of a switch, contactor and fuse unit with the fuse unit connected in series between the switch and contactor. U.S. Pat. No. 5,822,164 entitled xe2x80x9cPre-Engineered Combination Starter for Multiple Motor Sizesxe2x80x9d describes the use of an electronic overload relay, a contactor and a fuse for motor protection.
The use of a positive temperature coefficient device in series with an electronic circuit breaker to provide improved circuit protection to circuits including electric inductive motors to prevent the circuit breaker trip unit from responding to over-current motor start-up current is described within a more recent U.S. Pat. No. 5,539,370 entitled xe2x80x9cInductive Motor Protective Circuit Breaker.xe2x80x9d
It has since been determined that adequate short circuit protection can be provided to low ampere-rated electric switches having smaller-sized moveable contact arms and contacts by the use of an electronic trip circuit for motor overload and phase loss conditions in induction motor protective circuits, a fuse for short circuit interruption, and a motor contactor unit for turning the motor ON and OFF. A deterrent to the use of replaceable fuses with electronic circuit protective devices is the inconvenience of locating the fuse and replacing the fuse before re-closing the electric switch contacts.
A further deterrent to the use of a motor contactor unit in combination with electronic trip units is the requirement of supplemental power for operation of the electronic trip units when the switch contacts are opened. This is because electronic trip units compensate for thermal memory effects through software algorithms. Thermal memory, i.e., operating memory, is defined in NEMA standard ICS2 as the cumulative heating effect in a motor circuit resulting from motor operation or overload, and the cooling effect after the motor circuit is deenergized. However, the ability to compensate for thermal memory is lost in prior art electronic trip units when power is interrupted.
A recent attempt to use the electronic trip unit in a xe2x80x9csmartxe2x80x9d circuit breaker incorporating an application specific integrated circuit (ASIC) microprocessor in combination with a motor contactor unit is found in U.S. Pat. No. 5,657,193 entitled xe2x80x9cElectronic Control Module for Motor Controller Unitsxe2x80x9d wherein a smart circuit breaker trip unit is interconnected with a contactor unit to employ the current sensing and circuit interrupting function of the circuit breaker in a motor protection circuit.
Thus there is a particular need for a molded case combination starter having an electronic trip unit for motor overload conditions, a fuse for short circuit interruption and a manual disconnect switch.
The molded case combination starter housing has an enclosure which includes a motor contactor control circuit within one enclosure for long motor overload, and a replaceable fuse unit within a separate enclosure for handling short-circuit type faults. Upon the occurrence of a short circuit within the protected circuit, the fuse rapidly responds to interrupt the short circuit. The fuse is conveniently replaced before power is restored to the protected circuit.
In an exemplary embodiment of the invention, a manual disconnect switch is provided. The disconnect isolates the motor during a maintenance operation, e.g., changing of the fuse.